The instant application should be granted the priority dates of Jul. 9, 2010, the filing date of the corresponding German patent application 10 2010 026 731.7.
The present invention relates to a method and an apparatus for moving along a tube extrusion line having an extruder, a calibration and cooling unit, and a withdrawal unit.
Tube extrusion lines are provided with an extruder having a die, a so-called casing head, a calibration and cooling unit and possibly further cooling tanks, as well as a withdrawal unit. During normal operation, the extruded tube is pulled through the extrusion line at the required velocity by the withdrawal unit, which is generally embodied as a caterpillar-type withdrawal mechanism.
During the moving along, there is generally no tube in the extrusion line. A starting or dummy tube must therefore be used. A dummy tube must be “threaded through” all of the devices from the rear end of the extrusion line until its front end extends out of the calibration and cooling unit. It can then be connected, for example by fusing or heat sealing, with the molten tube exiting the casing head. After the production of this connection, a pulling force is applied to the dummy tube by the withdrawal unit, so that ultimately the newly produced tube, which is connected to the dummy tube, enters the withdrawal unit. The task of the dummy tube is therewith finished.
This method of proceeding known from the state of the art has a number of drawbacks. For example, the dummy tube must as closely as possible have the same diameter as does the tube that is to be produced, so that the final seal of the calibration and cooling unit, which is sealed relative to the atmosphere, rests against the dummy tube and immediately an underpressure can be employed in the calibration and cooling unit. Otherwise, the number of rejects of newly produced tubes would increase. In addition, the produced tube and the dummy tube should as much as possible be made of the same material in order to be able to optimally produce the fused connection. A further drawback during the moving along of an extrusion line using a dummy tube is that the dummy tube must be “threaded” from the rear through the entire length of the extrusion line. In so doing, components of the unit can be damaged. Furthermore, this “threading process”, where the metric weight of the dummy tube is great, is, in the truest sense of the word, very difficult.
The aforementioned drawbacks are avoided with a moving along system that is disclosed in DE 202 21 671 U1. With this system, a so-called coupling head is utilized. For the moving along of the extrusion line, this head projects out of the calibration and cooling unit in the direction of the casing head. The coupling head has a cylindrical projection with which the molten tube exiting the casing head can be connected, for example by fusing or clamping. The coupling head is connected to a pulling mechanism. This pulling mechanism is provided, for example, with a cable that is guided from the coupling head through the entire extrusion line, and is connected at the rear end with a winch. After the connection of the molten tube exiting the casing head with the coupling head, a pulling force is applied to the coupling head, and hence to the produced tube, by means of the pulling mechanism. As soon as the coupling head has passed the withdrawal unit, the latter takes over the further withdrawal of the produced tube in the tube extrusion line.
During the moving along, an overpressure is produced by the supply of compressed air in the interior of the molten tube between the casing head and the coupling head. Since atmospheric pressure still exists in the calibration and cooling unit, the molten tube is pressed against a calibration device, for example a calibration sleeve, due to a pressure differential. As a result, the produced tube obtains the desired outer diameter already at the beginning, so that rejects during the moving along can be reduced. After the coupling head has passed a seal disposed at the outlet of the calibration and cooling unit, this seal automatically or in a positively controlled manner rests sealingly against the produced tube, so that a vacuum is generated in the calibration and cooling unit, and the supply of compressed air into the interior of the molten tube can be shut off. Since now furthermore there is a pressure differential between the interior of the tube and the calibration and cooling unit, the calibration of the tube can be continued in an uninterrupted manner.
A drawback of the technology described in DE 202 21 671 U1 is that a connection between the molten tube exiting the casing head and the coupling head must furthermore be established. In addition, care bust be taken that the pulling cable extend fairly centrally through the extrusion line in order to prevent tilting or twisting of the coupling head.
DE 10 2005 028 085 A1 describes an apparatus for the connection of two tube ends. With this moving along technology, again a dummy tube is utilized, although in one embodiment this dummy tube is not connected directly with the molten tube, but rather via a counter piece about which the molten tube is disposed. The counter piece is connected via a traction eye and a cable with an engagement mandrel, which can be fixed in position in the dummy tube via expansion means.
It is an object of the present invention to provide a method and an apparatus for the moving along of a tube extrusion line that operate entirely without traction means and hence avoid the drawbacks thereof.